The present invention is directed to a paper web cut-off device comprising a frame part and blade means mounted on the frame part, the blade means being disposed to accomplish cross-cutting of the paper web, in addition to actuating means for producing, on the blade means, the force required in the paper web cutting operation.
In paper web handling apparatus, e.g. unreelers, the speed of the paper web is relatively high, i.e., on the order of 1000-2500 m/min. The width of the paper web is, as a rule, on the order of 4-8 m. It is necessary to cut off the paper web for splicing on the unreelers.
Paper web cut-off devices known in the art are of such design that a plurality of compressed air cylinders situated at intervals have been arranged to act on the cutting blade of the cut-off device. After cutting, the compressed air cylinders return the cutting blade to its original position. However, with this design of the prior art, high enough speed of the cutting blade cannot be achieved, and an excessive delay is therefore incurred in the cut-off operation, with the result of cutting-off at an incorrect point. In other words, the cut-off is inaccurate regarding location. The slow motion of the cutting blade also results in improper cutting of the paper web.
Presently, a paper web cut-off device is commonly required to have an accuracy of about 0.3 m. This means that the delay counted from the cut-off command, must be such that the cut-off error is within the specified limit of accuracy, 0.3 m. In the design of the prior art in which a plurality of compressed air cylinders spaced from one anotheer is used, the time lag is on the order of 0.25 sec. With present-day web speeds, this corresponds to a distance of about 5 m. on the web. This implies that, if a 10% error is allowed in the cutting, an accuracy of the cut-off device of the prior art is no better than on the order of 0.5 m.